Photolithography process is very important for pattern definition in semiconductor technologies, in which the accuracy and stability of photolithography directly affect product quality. In particular, with the device size being decreased, the challenge to photolithography is increased, and slight variation in pattern exposed leads to a great error and effect. In addition, device manufacture usually involves a stack of multiple films, and therefore aligning the under-layer pattern is performed before exposure for each film pattern. Thus, the critical dimension (CD) loss of patterns or pattern deviation hinders alignment accuracy, and results in product defects and the rework rate of photolithography being raised.
Many factors, such as product type, equipment condition and human factors, have obvious influence on photolithography process, and therefore accurate exposure conditions have to be adjusted constantly in response to actual variation. At present, an automatic compensation adjustment method is generally used to deal with the CD loss and pattern deviation problems for exposure variation each time.
The automatic compensation adjustment method utilizes the difference between a real measurement value and a predictive value recorded for each run of exposure process, and combines a last exposure adjustment value with exposure variation to calculate another exposure adjustment value for a next run of exposure process. The exposure adjustment value calculated is used to offset exposure variation automatically before each exposure operation is performed.
The automatic compensation adjustment method used popularly is called “Photo Feed-Back System” (PFBS), and the exposure adjustment value calculated is the PFBS parameter. The PFBS is just a single formula, and has only one PFBS parameter for all kinds of products. The real measurement value and the PFBS parameter operated are used as databases to evaluate the next PFBS parameter before the next run of exposure operation is performed.
But the PFBS aforementioned is only suitable for use with one type of product, and the database includes different types of products if miscellaneous products exposed are alternately added in operation. The exposure condition is changeable in practice and is affected by differences in multiple products; there is thus an error in the exposure adjustment value evaluated if the preceding database is directly used without consideration for product differences.
In addition to a host product, various minor products are sometimes added alternately to general operations, but exposure error adjustment uses the same PFBS for host product and minor products separately. Referring to FIG. 1, exposure adjustment calculation for host product and minor products is performed separately. The Y-axis in FIG. 1 shows the PFBS parameter, and the X-axis shows recorded exposure date. Various minor products are considered miscellaneous products. The dashed line shows variation in the PFBS parameter of miscellaneous products, and the solid line shows variation in the PFBS parameter of the host product. The same PFBS calculation formula is used for host product and miscellaneous product separately, and the PFBS of host product and the PFBS of miscellaneous product are independent.
As shown in FIG. 1, the solid line curve 110 and the solid line curve 130 represent the PFBS parameter variation of first-run and second-run for host product respectively. The dashed curve 120 and the dashed curve 140 represent the PFBS parameter variation of first-run and second-run for miscellaneous products, respectively.
The end-point PFBS parameter of the first-run for host product (the end-point of the solid line curve 110) is chosen as the starting PFBS parameter of the second-run for host product (the starting point of the solid line curve 130) directly; that is, the starting point of the solid line curve 130 is the same as the end-point of the solid line curve 110. Similarly, the starting PFBS parameter of the second-run for miscellaneous product (the starting point of the dashed line curve 140) is also chosen as the end-point PFBS parameter of the first-run for miscellaneous product (the end-point of the dashed line curve 120) directly; that is, the starting point of the dashed line curve 140 is the same as the end-point of the dashed line curve 120.
The PFBS parameter evaluated has an error if the PFBS calculation only considers exposure condition of the same product last processed as the database without considering exposure conditions of other different products inserted.
Actually, there are various multiple products in operation, and the variation of exposure deviation and CD loss are affected by different products, and therefore operation for host product and minor miscellaneous product cannot be considered independent respectively. The PFBS used nowadays still experiences inaccuracy in exposure error adjustment in photolithography for multiple products.